A thermometer is an instrument used to measure temperature. Most modern thermometers tend to be a hollow tube of glass marked with a scale and filled with a liquid, either mercury or alcohol. The thermometer works because there is a constant ratio between the amount the liquid expands and the increase in temperature. The standard mercury thermometer was invented by German physicist Gabriel Fahrenheit in the early 1700s. (He devised the temperature scale that bears his name.) The liquid is stored in a reservoir at the base of the thermometer and the column is marked off with an appropriate scale. Strictly speaking, the linear relationship between the temperature and expansion of these liquids does not hold true. For greater accuracy and reliability, the liquid should be replaced with gas. A gas thermometer has the column filled with gas at a low pressure and there is a weight at the top of the column to maintain a constant pressure.
A gas thermometer has an advantage in that it can be used over all temperatures encountered. A mercury thermometer cannot be used below -38.2°F (-39°C) as this is the temperature at which mercury freezes. An alcohol thermometer will remain liquid down to a temperature of -175°F (-115°C) although it boils at 158°F (70°C).
Other types of thermometers includes the thermocouple, which is a pair of wires or semiconductors joined at both ends. One junction of the two materials is at a fixed temperature and the other is at the temperature to be measured. This results in the flow of an electric current and for a known pair of materials, the current flowing is directly proportional to the temperature difference. By connecting the system to a suitably calibrated ammeter, the temperature can be read. A similar system operates with a bimetallic strip. Two different types of metal are bonded together and the temperature can be calculated using the difference in the expansion shown by each metal, this forces the strip to bend one way or the other, the degree of bending being proportional to the temperature.
Another type of thermometer now used is known as the Galileo thermometer. This is named after the Italian scientist Galileo Galilei. This type of thermometer utilizes the principle that different liquids and mixtures of liquids have different densities at different temperatures. Sealed glass balls of mixtures of organic liquids are placed in a column of alcohol. The balls rise or fall depending upon the temperature and the appropriate temperature is engraved on the glass ball. It must be stressed this type of thermometer works on principles first worked out by Galileo, it was not invented by him nor was it in use during his time. Of the types of thermometer discussed this is the least accurate. The most accurate thermometer is the constant pressure gas thermometer.
Some thermometers are designed for specific purposes and consequently they operate over a very small range of temperatures. For example, a clinical thermometer is made to work only a few degrees either side of body temperature. It has a very fine capillary tube internally so that small changes in temperature can readily be seen. Another adaptation the clinical thermometer has is a small constriction in the tube so the mercury or alcohol cannot return to the bulb until it is reset. This allows the temperature to be viewed once the thermometer has been removed.
This is the complete article, containing 559 words
(approx. 2 pages at 300 words per page).
